A process transmitter generally includes a transducer or sensor that responds to a process variable. A process variable generally refers to a physical or chemical state of matter or conversion of energy. Examples of process variables include pressure, temperature, flow, conductivity, pH and other properties. Pressure is considered to be a basic process variable in that it can be used to measure flow, level and even temperature.
Pressure transmitters are commonly used in industrial processes to measure and monitor pressures of various industrial process fluids, such as slurries, liquids, vapors and gases of chemical, pulp, petroleum, gas, pharmaceuticals, food and other fluid-type processing plants. Differential pressure transmitters generally include a pair of process pressure fluid inputs which are operably coupled to a differential pressure sensor (within the transmitter) that responds to the difference in pressure between the two inputs. Differential pressure transmitters typically include a differential pressure sensor operably coupled to a pair of isolator diaphragms. The isolator diaphragms are positioned at the process fluid inlets and isolate the differential pressure sensor from the harsh process fluids being sensed. Pressure is transferred from the process fluid to the differential pressure sensor through a substantially incompressible fill fluid carried in a passageway extending from the isolator diaphragm to the differential pressure sensor.
Differential pressure sensors generally include a movable diaphragm that has a first side coupled to a first pressure, and a second side coupled to a second pressure. The difference between the pressures generates a net displacement on the movable diaphragm. The diaphragm has an electrical characteristic, such as capacitance or resistance that varies with the displacement. The electrical characteristic can then be monitored, or otherwise measured, as an indicator of the differential pressure. Differential pressure sensors are useful in many applications. However, they are often found in applications where process fluid flow is measured. In these applications, a differential pressure producer is disposed within a process fluid conduit, such as a pipe, and fluid flow through the producer generates a differential pressure. The differential pressure generated across the producer is then mathematically related to process fluid flow through the conduit.
While the differential pressure itself may be of any magnitude, depending on the process fluid flow, viscosity, density, et cetera, the actual line pressure of the process fluid within the conduit can vary independently of the differential pressure. For example, a process fluid of relatively low density, flowing through a slight obstruction may only generate a slight differential pressure. However, the overall pressure within the flow conduit may be extremely large. Accordingly, differential pressure sensing systems are generally specified the maximum differential pressure that can be transduced, as well as the maximum line pressure to which the system can be exposed. Such systems are generally designed to accommodate at least some pressure excursions beyond maximum stated line pressures. These excursions are known as overpressure events. The manner in which such differential pressure systems respond to and recover from such overpressure events is extremely important. For example, if the overpressure event ruptures, or otherwise deteriorates process fluid couplings within the sensing system, the ability of the sensing system to continue to operate is destroyed. Additionally, if plastic deformations occur within the differential pressure sensing system, a systemic error may be introduced from that point forward, which error will affect all subsequent differential pressure measurements.
An overpressure event for such a system may cause the movable diaphragm of the sensor to fully engage a wall of the sensing chamber. In such situations, the interior of the differential pressure sensor itself is fully subject to the line pressure during the overpressure event. While this is clearly undesirable, it is even more undesirable for semiconductor-based pressure sensors. These semiconductor-based differential pressure sensors typically employ brittle materials, such as semiconductor materials, and are built from a stack up of layers of semiconductor material. They are typically bonded together to form the overall sensor, but it is known that such sensors are not able to withstand significant tensile forces on the layer interfaces.
A process fluid differential pressure transmitter with better responses to overpressure situations would advance the art of sensing differential pressure process fluid. Additionally, such a transmitter may allow for operation in more demanding application, and/or provide longer operating lifetimes.